In a typical operation of a gas turbine engine, the combustor generates high temperature combustion gases that pass through a turbine having a plurality of airfoils. In order to protect these airfoils from the extreme temperatures of the combustion gases, a variety of cooling techniques have been developed. For instance, a plurality of cooling holes may be formed in an outer surface of the turbine airfoil. These cooling holes are adapted to communicate a cooling fluid (e.g., air or steam) from an inner reservoir within the turbine airfoil to the exterior surface of the turbine airfoil. The high velocity of the high temperature combustion gases causes the emitted cooling fluid to wrap over the outer surface of the turbine airfoil and create a thin, protective film layer of cooling fluid between the airfoil outer surface and the high temperature combustion gases. Surface coverage and uniformity of this protective film is essential in improving long term durability and structural integrity of the turbine airfoil.
Referring to FIG. 5A, the second, exterior surface 24A of a prior art airfoil having a plurality of cooling holes 30A is depicted. In order to achieve a consistent protective film layer across the airfoil and maintain the structural integrity of the airfoil, adjacent cooling holes 30A are spaced apart by a spacing wall 58A. The openings of the cooling holes 30A, on the second, exterior surface 24A of the airfoil, are separated by a distance 60A. The distance 60A has been a function of tolerances required to maintain the spacing wall 58A as a unitary member between adjacent cooling holes 30A for the entire distance between the first, interior surface (not shown) and the second, exterior surface 24A. In other words, the distance 60A has been a function of the tolerances required to prevent adjacent cooling holes 30A from intersecting within, or at the surfaces, of the airfoil wall when they were formed. Prior art cooling holes 30A typically included a diffusing section adjacent to the second, exterior surface 24A. Therefore, the distance 60A was dependent on variations of casting surface profile, electrode plunge depth, electrode profile, and part positional tolerances due to opposing sides of the prior art diffusion section being at an angle relative to each other. The distance 60A of prior art diffusion holes are significantly greater than the present invention to account for these previously uncontrollable dependencies. In addition, the diffusing sections of the prior art cooling holes 30A would emit cooling fluid from within the turbine airfoil in a direction where a portion of the emitted cooling fluid from one cooling hole 30A would intersect with a portion of the emitted cooling fluid from an adjacent cooling hole 30A. The intersection of these portions of emitted cooling fluids would cause increased turbulence in the emitted cooling fluid, which is undesirable for forming a uniform protective layer of film between the hot combustion gases and the second, exterior surface 24A. Further, having a diffusing section of the cooling hole 30A at the exterior edge of the cooling hole 30A required an increased distance 60A and consideration of all applicable tolerances in order to avoid one cooling hole 30A intersecting another cooling hole 30A within the wall of the airfoil, or more particularly, at the second, exterior surface 24A of the airfoil.